Long Polling vs WebSockets

Long polling and WebSockets are two approaches to achieving real-time communication in web applications. Long polling simulates real-time by keeping HTTP requests open until new data arrives. WebSockets establish a persistent, full-duplex connection.

How Long Polling Works

The client sends an HTTP request to the server. Instead of responding immediately, the server holds the request open until new data is available or a timeout occurs. When data arrives, the server responds, and the client immediately sends a new request. This creates a near-real-time push mechanism using standard HTTP.

How WebSockets Work

The client initiates an HTTP request with an Upgrade: websocket header. The server accepts, and the TCP connection is repurposed for bidirectional message passing. Both sides can send messages at any time with minimal overhead.

Why Long Polling Still Exists

Despite WebSocket's technical superiority, long polling persists because it works everywhere. Corporate firewalls, HTTP proxies, and older load balancers that block WebSocket upgrades still handle long polling perfectly. It is a reliable fallback.

Many real-time libraries (Socket.IO, SignalR) automatically fall back from WebSockets to long polling when the upgrade fails.

The Performance Gap

For high-frequency updates, the gap is significant. Each long-polling cycle requires a new HTTP request with full headers (500-2000 bytes). WebSocket messages have 2-14 bytes of overhead. At thousands of messages per second, this difference matters.

For more on real-time patterns, see our concepts library and system design interview guide. Explore pricing for practice.

The Bottom Line

Use WebSockets as the default for real-time features. Fall back to long polling only when infrastructure constraints prevent WebSocket connections.